The powerful methods of three-dimensional x-ray crystal structure analysis will be used to determine quantitatively the most basic aspects of porphyrin stereochemistry. Metalloporphyrins to be studied include derivatives designed to mimic certain stereochemical features of the porphinato core or the coordination group that have been suggested to be important in the cooperative binding of oxygen and other ligands to hemoglobin and metal-substituted hemoglobin and metal-substituted hemoglobins. These studies should yield a better understanding of the fine details of the stereochemical basis of cooperative effects. Other studies to be undertaken include the synthesis and structure of metalloporphyrins further coordinated by small molecules or ions, additional manganese (II) porophyrins and metalloporphyrins with unusual magnetic properties (spin states).